Unzen Scientific Drilling Project (USDP) is a six-year term international project started in April 1999, co-sponsored by the Japanese Government (MEXT) and International Continental Scientific Drilling Program (ICDP). More than 20 institutes nationwide and abroad are participating the project, and AIST is serving as one of the principle research centers. The highlight of this project, to penetrate the magma path (conduit) of the Heisei eruption in the middle of the mountain, was successfully completed after reaching the conduit of the Heisei magma in July 2004.
At Unzen volcano, pyroclastic flows caused by the collapse of lava domes occurred frequently during the Heisei eruption between 1991 and 1995, causing serious damage and 44 casualties. The surveys on these eruptions produced much observational data on such as earthquakes, uplifts, and deformations of mountains, which provided us with a detailed model of the ascent and eruption process of magma.
USDP is aiming to understand the growth history, subsurface structure and magma ascending mechanism of Unzen Volcano not only by scientific drillings but also by related geological, geophysical and geochemical studies. In the project, we planned to drill a hole to reach the conduit of the Heisei eruption in order to clarify the ascending and degassing process of magma and understand the mechanism of an eruption. The driling work began in February 2003 from the site on the northside slope of Unzen volcano at the altitude of 850 meters. Work was suspended several times, but we were able to reach the Heisei conduit by making some revisions after drilling approximately 1.3 kilometers horizontally and approximately 900 meters perpendicularly, the total drilling distance was 2000 meters.
There was a conduit zone near the area right under Mt. Fugen at around the sea level, where old and new conduits including the Heisei conduit were concentrated within approximately 500 meters. The temperature in the conduit zone is approximately 200°C, lower than expected before the drilling, which is assumed to have resulted from quick cooling due to hydrothermal activities.
Future extensive research is expected to provide the details of the ascending process of magma.